Since the successful development of high activity catalysts for polyethylene in 1970's, there are siginificant changes in the polyethylene industry. In recent 20 years, there has been rapid progress with respect to the researches into the catalysts for ethylene polymerization and three systems in terms of the composition of the catalysts, i.e. chromium type high activity catalysts, Ziegler-Natta type Mg-Ti high activity catalysts and metallocene type catalysts,have been formed. Among them, Mg-Ti high activity catalysts are very important in the field of catalysts for polyethylene due to their excellent performance. The processes for preparing them have been developed from the co-grinding process as well as the suspending-impregnating process to the advanced chemical reaction process utilized at present, and in this regard, there are many patents issued. However, these patents owned by different companies are largely different and each of them possesses advantages and disadvantages.
For example, Japanese Patent Application Laid-open No. Sho 51-64586 discloses a catalyst for polymerizing olefins, comprising a solid catalyst component (A), prepared by reacting metal magnesium with an alkyl halide in the presence of an electron donor compound to form a magnesium compound having a ratio of halogen/magnesium of 1-2 and containing no magnesium-carbon bond, reacting said magnesium compound with an organometallic compound or halide of Group III metal of the Period Table, and then reacting the resulting product with a titanium compound, and an organometallic compound of Group I-III metal of the Period Table, in which titanium is substantially tetravalent. The application specifically describes in the Examples that small amount of ethanol or other alcohols is used as the electron donor compound so as to facilitate the reaction between metal magnesium and n-butyl chloride to form white precipitate. The disadvantage of the catalyst is that during the preparation of the solid catalyst component (A), it is necessary to use titanium tetrachloride in an amount largely higher than that to be supported, which give rises a low rate for supporting titanium and leads to equipment errosion.
Chinese Patent Application No 93118601.3 discloses a titanium catalyst component, prepared by reacting anhydrous magnesium chloride with an alcohol compound containing at least 6 carbon atoms, such as 2-ethylhexanol, to form a uniform solution, to which is added an organic aluminum compound to form a precipitate, and then impregnating said precipitate with large amount of titanium tetrachloride, in which titanium is substantially tetravalent. The titanium catalyst component has higher catalytic activity when used for polymerizing ethylene, and the products obtained have uniform partice size and high bulk density. However, during the preparation of the titanium catalyst component, it is required that the magnesium compound be dissolved at elevated temperature of 120-140.degree. C. and higher alkane solvent such as decane be used in large amount, which results in high cost and there is a problem with respect to the recovery of the solvent. In addition, it is necessary to use titanium tetrachloride in an amount largely higher than that to be supported during the supporting of titanium, which leads to equipment errosion.
Chinese Patent Application No. 93103264.4 discloses a process for process for preparing a catalyst component containing titanium, comprising the step of dissolving magnesium chloride in a mixture of an organic phosphate and an alcohol to form a uniform solution, to which a titanium compound is added in the presence of an coprecipitator such as an organic carboxylic anhydride to precipitate the catalyst component. The catalyst obtained according to this process is characterized in high catalytic activity, excellent properties of the resultant polymer products, low amount of solvent used and improved productivity. However, during the preparation of the catalyst component, it is required that the temperature at which the solution of magnesium compound is mixed and reacted with the solution of titanium tetrachloride be from -5.degree. C. to 10.degree. C., which is a severe operation condition and is not suitable for production and application in industrial scales.
The present inventors proposed a catalyst in Chinese Patent Application No. 93112345.3, which is prepared by using powdered magnesium as raw material to form a complexed magnesium halide carrier having reducing capability and then supporting active components onto said carrier by impregnating, with more than 80% of titanium being trivalent. Said catalyst is advantageous for a simple preparing procedure, suitable initial activity of polymerization reaction, smooth and stable reaction, high catalytic activity, good morphology of the resulting polymer particles and lack of fine particles. However, as a catalyst for producing polyethylene by a slurry phase process, the catalyst is insufficient in its catalytic activity.